This function changes the focus window.

#define INCL_WININPUT /* Or use INCL_WIN, INCL_PM, Also in COMMON section */
#include <os2.h>

HWND     hwndDeskTop;    /*  Desktop-window handle. */
HWND     hwndNewFocus;   /*  Window handle to receive the focus. */
ULONG    flFocusChange;  /*  Focus changing indicators. */
BOOL     rc;             /*  Success indicator. */

rc = WinFocusChange(hwndDeskTop, hwndNewFocus,
       flFocusChange);